A mass storage device, such as a hard disk drive, utilizes a spindle motor to rotate platters that store information. The spindle motor generally rotates the platters at a constant angular speed while electromagnetic heads read from or write to circular tracks on the platters.
A three-phase spindle motor may include a magnetic rotor and three electric coils. The three electric coils are associated with the three phases of the motor. A separate current, called a phase current, flows through each of the three electric coils of the motor. The rotor rotates in response to an electrical field created by the phase currents.
The phase currents flowing through the three electric coils may interact with the magnetic elements in the motor to produce acoustic noise. In some motors, each electric coil cycles through three states: each electric coil can be held at ground, driven to some positive voltage, or floated. As a result, the phase currents flowing through the three electric coils have very abrupt transitions. Because torque is proportional to current, the abrupt changes in the phase currents cause the torque also to change abruptly. If the harmonics in the torque waveform excite mechanical resonances, the motor structure may vibrate and generate audible noise, which is not desirable.
To reduce the harmonics of the torque waveform, the motor can be driven with sinusoidal phase currents. Driving voltages can be applied to the electric coils of the motor to generate sinusoidal phase currents through the electric coils. However, in order to effectively operate the motor, each phase current should be aligned in phase with its corresponding back electromotive force. When each phase current is properly aligned with its corresponding back electromotive force, the motor produces maximum torque. Given a method of detecting phase error, a phase-locked loop can utilize current feedback to adjust the motor's commutation to achieve the proper alignment.
In some systems, a phase detect circuitry can recover the back electromotive force waveform from the undriven electric coil and then generate a signal proportional to the phase error. However, in a spindle motor driven with sinusoidal currents, the electric coils are constantly driven, so a phase detect circuitry cannot directly measure the back electromotive force to generate a signal indicative of phase error.